The present invention relates to separation of liquid droplets from a gas stream, particularly in production of oil and gas. More precisely, the present invention relates to an inlet device intended for use in gravity separators designed typically for the removal of liquid droplets from a gas stream.
During production of oil and gas from a subterranean reservoir, the well stream will normally contain oil, gas, water and some solid particles. In order to separate the various fluids and solids, a dedicated process system for the well stream is constructed. The separation is made in several stages, where the “bulk separation” of the various phases is carried out by gravity forces alone where the immiscible fluids are separated based on difference in densities, and the “fine separation” or purification is often done utilizing centrifugal forces and inertial forces together with the gravity force.
A challenge appearing in many separation stages is to remove liquid droplets from a gas stream where the liquid content in the gas is low, typically less than 3 vol % of the total volumetric flow. It is of outmost importance to remove most of this liquid in order to protect downstream equipment such as compressors and dewatering equipment where only traces of liquid may create operational problems.
In the following, separators dedicated to separate gas/liquid mixtures containing less than said 3 vol % liquid is denoted gas scrubbers.
The gas scrubbers will often be a vertical vessel but may also be a horizontal vessel or a combination of a vertical and horizontal vessel. Inside the gas scrubber vessel, the separation often takes place in several stages. First, the gas enters through an inlet nozzle, which—for vertical oriented scrubbers—can be located approximately at the middle of the scrubber in the vertical direction. At the inlet nozzle a momentum breaker plate, a vane diffuser or any device can be located in order to distribute fluids across the separator cross-sectional area. Already here, the largest drops are separated and falling down onto the liquid reservoir in the lower part of the separator.
The gas will flow upwards into a calm zone, or deposition zone, where further droplets due to gravity falls down onto the liquid surface below, alternatively deposits on the separator wall and drain downwards along this.
Close to the top of the separator, the gas is forced to pass through droplet separation equipment of known technology. There are mainly three categories of droplet separation equipment; mesh pad, vanepack and parallel arranged axial flow cyclones. Because of the pressure drop across the droplet separation equipment, the separated liquid is normally drained down to the liquid reservoir through a said drainpipe, whose lower end is submerged in the liquid reservoir.
It is important that the separator inlet device is correctly designed relatively to the separator cross sectional area in order to remove as much liquid as possible to minimize the amount of liquid fed to the demisting equipment. This is particularly important for vertical scrubbers and contactor columns utilized to remove aqueous vapor from a gas stream. Too much liquid fed to the demisting equipment caused by poorly designed inlet devices and/or too small scrubber diameters relative to the gas flow rate are the main reasons for malfunction experienced on a large number of scrubber installations. Most inlet devices of known technology uses gravity forces alone to separate liquid in the scrubber inlet compartment, giving stringent limits to the gas velocities before considerable amounts of liquid is following the gas to the demisting equipment. Inlet cyclones have successfully replaced earlier used inlet devices in modern 2- and 3-phase separators where the liquid content is high, typically above 5 vol %, but inlet devices described as vane diffusers still represents state of the art technology in vertical gas scrubbers where the liquid fraction is less than 3 vol %. Lately, cyclone inlet devices are also applied in gas scrubbers. However, some operational problems associated with cyclone inlet devices used in gas scrubbers will be explained in the following.
A more thorough description of the prior art is given in the specific part of the description with reference to drawings.